The conventional way to manufacture "semi-finished" steel products, such as rounds, slabs, blooms, and billets, is to pour molten steel into an ingot and then roll the ingot down into a round, slab, bloom, or billet in a rolling mill. The semi-finished products are then made into bars, tubes, sheet, strip, or the like, which are called "finished products."
It may require up to twenty-five or more passes through various rolling mills to transform an ingot into a semi-finished product. The reduction in cross-sectional area from an ingot to a semi-finished product is at least four to one. A great deal of energy, as well as expensive capital equipment, is thus required to reduce the ingot to semi-finished product forms.
A more modern technique for making slabs and other semi-finished products is the continuous casting process whereby molten steel is poured into a tundish, from there into bottomless, cooled vertical molds, and then withdrawn by rolls or other mechanisms in a continuous length. Pieces are cut off from this continuous length to give slabs, blooms, or billets as desired, in accordance with the shape of the vertical mold. Although deceptively simple in principle, this technique has, in practice, many inherent difficulties. Continuous casting equipment is bulky and requires a large amount of space for each installation. The capital investment is enormous, and the process is not suitable for low volume production.
Although slabs and billets can be cast by the continuous casting process, the casting of rounds by continuous casting has not proved satisfactory because rounds have the least surface area per unit of volume and are difficult to cool and otherwise handle in a satisfactory manner in a continuous casting process.
Another technique for making slabs, particularly stainless steel slabs, is the bottom pressure pouring method described in may U.S. Pat. No. 3,196,503. According to this technique, a ladle filled with molten steel is placed in a pressure vessel which is sealed with a lid. A pouring tube extends through the lid down to within approximately 4 to 6 inches (10 to 15 cm) from the bottom of the ladle. The top part of the pouring tube is mechanically connected to the filling end of the slab casting mold. Air pressure within the vessel causes the molten steel to rise through the pouring tube and enter the slab mold at the lower end, the mold being at a slight tilt. The equipment is expensive, and there are problems with inclusions, since the top portion of the slab to which inclusions normally rise is the first portion to become cool.
In early U.S. Pat. Nos. 298,662 and 319,779, it was proposed to make ingots in a mold with a falling bottom or blunger. As far as I know, these concepts never developed into commercial processes and never were capable of making semi-finished steel products. Ingots are generally much larger than semi-finished products and their surfaces do not have to be as smooth and free of defects.